Apparatus for imaging microvascular blood flow

ABSTRACT

An apparatus for measuring and imaging blood perfusion in tissue comprises a monochromatic laser light source, means for shaping the laser light beam, means for irradiating a section of the surface of the tissue with the laser light beam, means for collecting light scattered from the irradiated section, an image sensor comprising a plurality of photodetectors, each photodetector of the sensor being able to receive collected light from a predetermined sub area of the section of the tissue surface and produce a corresponding electrical output signal linearly related to the detected instananeous laser light intensity; means for processing the electrical output signals from the plurality of photodetctors, means for calculating the average Doppler frequency shift for each sub area from which scattered light is detected, means for producing an image of the blood perfusion in the tissue section irradiated from the processed output signals, and an image display means. The apparatus enables fast tissue blood perfusion image to sub second times.

The present invention relates to an apparatus for the measurement andimaging of particle movement and flow in fluids, particularly for themeasurement and imaging of blood flow in the small superficial bloodvessels of body tissue.

Blood flow in the small blood vessels of the skin performs an essentialrole in the regulation of the metabolic, hemodynamic and thermal stateof an individual and the condition of the microcirculation over bothlong and short time periods can reflect the general state of health. Thedegree of blood perfusion in the cutaneous microvascular structure oftenprovides a good indicator of peripheral vascular disease and reductionof blood flow in the microcirculatory blood vessels can often beattributed to cutaneous vascularisation disorders; so there are manysituations in routine clinical medicine where measurement of the bloodflow is important.

The microcirculation, its responses to stimuli, and its response totherapeutic regimes, were not open to routine continuous assessment andinvestigation until the introduction of the laser Doppler technique inthe 1970's and subsequent developments in the 1980's.

The technique depends on the Doppler principle whereby laser light whichis incident on tissue, typically the skin surface, is scattered bymoving red blood cells and undergoes frequency broadening. The frequencybroadened laser light, together with laser light scattered from statictissue, is photodetected and the resulting photocurrent processed toprovide a signal which correlates with blood flow.

Perfusion measurements using single and multiple channel fibre opticlaser Doppler monitors have been made on practically all tissues andapplied in most branches of medicine and physiology. The technique andits application has been described in numerous publications. Arepresentative selection of these are included in ‘Laser—Doppler BloodFlowmetry’, ed. A. P. Shepherd and P. Å. Oberg, Kluwer AcademicPublishers 1990 and also ‘Laser Doppler’, ed. G. V. Belcaro, U.Hoffmann, A. Bollinger and A. N. Nicolaides, Med-Orion Publishing Co.1994.

The application of these principles to measurements in themicrocirculation was described by M. D. Stern in Nature Vol 254, 56,March 1975, ‘In vivo evaluation of microcirculation by coherent lightscattering’; M. D. Stern et al 1977 ‘Continuous measurement of tissueblood flow by laser ‘Doppler spectroscopy’ Am J. Physiol 232: H441-H448;and subsequently in U.S. Pat. No. 4,109,647.

For some clinical applications, such as plastic surgery and woundhealing, point measurements using optic probes attached to the skin areseverely limited and this has prevented widespread application in theseareas. Three reasons for this are: point to point variation (spatialvariability) requiring several readings to give reliable measurement,contact between the probe and the tissue surface, and interference fromfibre movements which degrade the measurements.

These problems have been mainly overcome by the development of laserDoppler scanners which map perfusion over an area of tissue, typically100 cm² and in some cases over 1000 cm², using a scanning laser beam andone or more photodetectors. EP-A-0282210 describes an apparatus formonitoring blood stream in the skin surface which employs a linearsensor comprising a plurality of light receiving elements to receive thelaser light reflected by the skin surface, memory means for storing theoutput signals from the light receiving elements and calculating meansfor processing these signals to derive information about the bloodstream. The blood stream velocity or distribution information maythereby be calculated and displayed. WO90/11044 describes a method ofdetermination of blood flow and an apparatus for use therein whichinvolves projecting a beam of laser light to move over a surface beneathwhich blood flow in a vessel or vascular bed is to be determined,collecting the reflected and scattered light, measuring a spectrum offrequencies in the collected light and determining from differences inthe frequencies the blood flow beneath the surface under examination.WO91/06244 describes a system which includes means for directing a laserbeam onto a body part to be examined and guided movement of the laserbeam through a series of measurement points over the body part inaccordance with a predetermined scanning pattern. The laser beam ishalted at each measurement point for a given time interval. Thesedevices have found many research applications and have generatedconsiderable clinical interest.

The present invention seeks to significantly reduce imaging times, insome circumstances to sub second times, and provide means to recordvideo and blood perfusion images of the same tissue site simultaneously.

In the present invention provision can be made in the apparatus toswitch between two or more monochromatic laser light sources ofdifferent wavelengths to enable laser Doppler blood flow measurements tobe made sequentially at the different wavelengths.

The differences between measurements at different wavelengths providesinformation on flows at different depths below the tissue surface andinformation on tissue and blood absorption of light for the differentwavelengths.

In the present invention provision can be made in the apparatus toirradiate the tissue surface simultaneously with monochromatic laserlight of different wavelengths and simultaneous detection of lightscattered from the tissue at the two or more laser light wavelengthsusing a similar number of image sensors and suitable optical filters.

The magnitude of a blood perfusion measurement, commonly termed ‘flux’,which is proportional to the product of average red blood cell speed andred blood cell concentration in an element of the volume of tissuesampled, is dependent amongst other factors on the imager/tissue surfacedistance.

Provision can be made in the apparatus to measure the distance usingeither a CCD camera video image or an intensity photo image recordedusing the image sensor and compensate in the image processing for thedistance dependence.

The present invention provides an apparatus for measuring and imagingblood perfusion in tissue comprising

a monochromatic light source;

means for shaping the laser light beam;

means for irradiating a section of the surface of the tissue with thelaser light beam;

means for collecting light scattered from the irradiated section;

an image sensor comprising a plurality of photodetectors, eachphotodetector of the sensor being able to receive collected light from apredetermined sub area of the section of the tissue surface and producea corresponding electrical output signal linearly related to thedetected instantaneous laser light intensity;

means for processing the electrical output signals from the plurality ofphotodetectors to produce processed output signals comprising;

measurements of the power spectrum of the photocurrents generated in thedetection of laser light scattered from static tissue and Dopplerbroadened laser light scattered from moving blood cells;

the calculated average Doppler frequency shift for each sub area fromwhich scattered light is detected;

the calculated blood volume concentration for each sub area from whichscattered light is detected;

the calculated blood perfusion for each sub area from which scatteredlight is detected;

measurements of the intensity of the detected scattered light for eachpredetermined sub area;

means for producing an image of the blood perfusion in the tissuesection irradiated from the process output signals; and

an image display means.

Compared with the systems disclosed in WO90/11044 and WO91/06244 thepresent invention provides fast tissue blood perfusion imaging using animage sensor which is either a linear or two dimensional photodetectorarray for detecting the Doppler shifted and Doppler unshifted scatteredlaser light, and signal processing done with the aid of large scaledigital signal processor integrated circuits processing themulti-channel laser Doppler signals in parallel. The present inventioncan be used in a scanning or non scanning mode to produce bloodperfusion images and/or multi-channel blood perfusion recordings. In thescanning mode the laser beam can be scanned across the tissue surface ata substantially constant speed with measurements made on the fly orstepped across and measurements made with a halted beam.

According to EP-A-0282210 a simple differential calculation is used foranalysing speckle pattern to generate blood flow information. Thus thebandwidth of signals processed is severely limited, the measurementresults in erroneous evaluation of blood flow in the irradiated tissue,and in addition the calculation does not provide a blood fluxmeasurement.

The present invention has a means to process the full spectrum ofphotocurrents associated with the detection of scattered laser lightfrom tissue and red blood cells moving in the microcirculation. Itprovides measurements of a wide spectrum of red blood cell (rbc) speeds,the average rbc speed, measurement of the number concentration of rbcsin each sub section of irradiated tissue, and measurement of rbc flux.Furthermore it enables the combination of the blood perfusion (flux)image with a colour video image of the surface of the tissue. Thiscombination of images, showing and recording the location, colour andanatomical details of the tissue surface under examination together withthe blood perfusion image, provide the clinician with a powerfuldiagnostic instrument.

The array comprising a plurality of photodetectors used in the presentinvention may be a linear array or a two dimensional array.

Either type of detector array can be used in a non scanning mode i.e.with a fixed area of tissue irradiated. Using a two dimensionalphotodetector array enables a relatively large area of tissue bloodperfusion to be mapped. Using a linear array the blood perfusion along awell defined line in the tissue surface can be recorded. For both typesof detector flow measurements can be made with either single shortduration laser light exposure, or repeated exposures or continuousexposure. Short single or repeated exposures have the advantage that theaverage laser power incident on the tissue surface is reduced comparedto continuous exposure and hence higher peak laser powers can be used ifnecessary. Repeated exposures and continuous exposure enable multiplemeasurements to be made from each sub area of tissue enabling temporalvariations of blood flow to be recorded in addition to the spatialvariations.

Both types of detector array can be used in a scanning mode with eithermeasurements made with the area or line of irradiation moving across thetissue surface and measurements made on the fly, or with the laser beamstepped across the tissue surface and measurements made with the beamhalted. The laser beam movements can be produced by a rotating mirror orbeam splitter or by direct rotation of the laser beam shaping means.

With a two dimensional array the area of tissue that can be mapped,without moving the beam or tissue surface positions, depends on a numberof factors but in particular the laser power density on the tissuesurface, the number of photodetector elements in the array, and thedesired spatial resolution for the measurements (the distance separatingcentres of adjacent sub areas of tissue). The time taken to measure theblood perfusion and compute the blood perfusion map, for a given numberof photodetector elements depends mainly on the bandwidth of thephotocurrents processed, the desired frequency resolution, and thecalculating speed of the signal processors used and the number used inparallel processing. Sub second frame rates are achievable for a 64×64element array.

For a 16×16 element photodetector array and a spatial resolution of thetissue sub areas of between 2 to 3 mm, the measurement area for a nonscan mode of operation is about 40×40 mm². Larger areas can be mapped byincreasing the number of array elements or reducing the spatialresolution, assuming that the detected laser power per element enablesan acceptable signal to noise ratio to be achieved for the detectedsignals.

The tissue area being mapped requires illumination with laser light to asufficiently high power density to achieve the required signal to noise.For efficient use of the laser light power a relatively uniform powerdistribution is needed over the irradiated tissue area; howeverconventional beam expanding optics results in an approximate Gaussianpower distribution resulting in a large fraction of the irradiated areabeing illuminated with low intensity light. An embodiment of the presentinvention uses a simple, inexpensive means of shaping a collimated laserbeam, or a diverging beam from a laser diode, to produce a circularcross section diverging beam which has approximately uniform powerdistribution over more than half the beam width. The beam shaping meansis a length of multi-mode optic fibre which has a high numericalaperture into which the laser light is focused at one end. The emitteddiverging beam at the end has the characteristics described. The shapedlaser beam is directed at the target area by simply pointing the fibreend in the appropriate direction.

Two dimensional photodetector arrays, which are suitable for themulti-channel laser Doppler measurements performed by the apparatus ofthis present invention, can have a high cost. Suitable linearphotodetector arrays can cost significantly less and can be used inplace of a two dimensional array for some blood perfusion measurements.In general too there are fewer photodetector elements in the array sothat the required laser power for the illuminating beam is less.

In the present invention when a linear photodetector is used the tissuesurface is illuminated with a line of laser light with separation ofhalf power points across the line width typically 0.5 mm to 1.0 mm.

The beam diverges from the optical line generator lens so that theeffective line length on the tissue surface depends on the lens/tissuesurface distance. To ensure acceptable signal to noise for the detectedsignals in one embodiment of the present invention, and resolutions ofbetween 1.0 mm to 2 mm, line lengths are limited to approximately 90 mmif a 64 element photodiode is used. Typical line rates of 10 Hz orhigher are achievable.

Repeated measurements with a fixed beam position enables temporalrecords of blood flow changes to be recorded for as many points alongthe line of laser light irradiation as there are elements in the array.

Scanning the line across the tissue surface in a step mode enablesmeasurements to be made for a large area of tissue in a matter of a fewseconds. Mapping times of approximately 6 seconds for a 64×64 pixelimage have been achieved with one embodiment of the present invention.

In a simple optical line generator the basic optical components are acollimator and a cylindrical lens which converts a beam with a pencilshaped beam cross section to a diverging beam with a line cross sectionand with a Gaussian power distribution profile along its length. A moreuniform power distribution is preferred to make efficient use of laserpower and reduce power dependent signal variations.

In one embodiment of the present invention, a commercially availableline generator is used, which generates a line which is uniform in powerwithin approximately ±10% over approximately 80% of its length.

The means for irradiating a section of the surface of the tissue withmonochromatic light includes means for shaping the laser beam, andincludes means for directing the laser beam onto a section of the tissuesurface to be examined; preferably by reflection from a hot mirror ifnear infra-red laser radiation, or a beam splitter if visible laserradiation or a combination of visible and near infra-red laserradiation. A mirror or beam splitter also provides a means for movingthe beam over the said tissue surface in accordance with a determinedscanning pattern. The means for scanning the laser beam over the tissuesurface in a predetermined pattern includes a means for determining andrecording the position of the beam at any point of the scan and meansfor halting the beam at a predetermined position or positions during thescan. If the apparatus is intended for use in a non scan mode only, thelaser beam can be shone directly onto the tissue surface.

The means are typically a dc servo motor or stepper motor together withhigh resolution shaft encoders to provide accurate and reliable means tomonitor the reflection position and hence the position of the laserradiation on the tissue surface.

For rapid scanning of the beam a constant speed mode of scanning can beused with measurements made on the fly. This avoids the need to allow amirror settling time for each measurement though some noise due tomovement artefact is introduced and some low Doppler shift frequencyinformation is lost. The mechanical scanning means is preferably a dcservo motor with Proportional, Integral and Differential control (PID)of mirror angular speed and position using standard control algorithmsand systems. Angular speeds of the order of 5 rev/min are typically usedfor a fast scan.

Laser light scattered by the tissue is preferably reflected from the hotmirror, if near infra-red laser light, or from the beam splitter, ifvisible laser light, and is then imaged onto the photodetector arraytypically via an objective lens. For an apparatus intended for use in anon scan mode a cold mirror can be used to reflect visible light andtransmit near infra-red.

Each of the photodetectors of the array receives reflected and scatteredlight from a particular sub section of the section of tissue examined.By processing the signals from all the detectors in the array a onedimensional or two dimensional image of blood perfusion in the tissuesection can be produced.

The means for irradiating a section of the surface of the tissue withtwo or more monochromatic laser light sources of substantially differentwavelength in sequence includes a means for selecting a laser wavelengthfor irradiating the tissue surface and a means for switching betweenlaser outputs of different wavelengths. These means are preferably anelectronically controlled optic fibre switch with the number of inputoptic fibres corresponding to the number of laser light sources used,and one common output fibre. Some laser sources emit useful laser lightpower at two or more significantly different wavelengths. If the powersand wavelengths are suitable for the laser Doppler measurements such alaser can be used without recourse to an optic fibre switch.

The means to irradiate a section of the system of the tissue surfacesimultaneously with laser light of two or more significantly differentwavelengths comprises a suitable laser or laser sources and a means tocombine the outputs of two or more lasers which is preferably an opticfibre coupler with the number of input fibres corresponding to thenumber of sources used, and one common output fibre.

The multi-wavelength laser light can be shone via beam shaping opticsdirectly onto the tissue surface for an apparatus designed for non scanmode operation only, or via a front silvered mirror or a beam splitterfor an apparatus that is intended for operation in a scan or non scanmode.

Where the multi-wave light is a combination of visible and nearinfra-red and is shone directly onto the tissue surface a cold mirror ispreferred as the means to both filter visible from near infra-red andreflect the scattered laser light onto the photodiode arrays. Filteringof infra-red light of one wavelength from infra-red of anotherwavelength, and visible from visible, is done with band pass opticfilters.

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is an outline schematic drawing of an example of an imager usinga 2-D photodetector array which can be operated in scanning ornon-scanning mode;

FIG. 2 is an outline schematic drawing of an example of non-scanningapparatus using a 2-D photodetector array;

FIG. 3 is an outline schematic drawing of an example of an imager usinga 1-D (linear) photodetector array which can be operated in scanning ornon-scanning mode;

FIG. 4 is a block diagram of a blood perfusion image and video imageprocessing and display system employed in an embodiment of the presentinvention;

FIG. 5 is a block diagram of a laser Doppler signal collection andprocessing unit employed in an embodiment of the present invention;

FIG. 6 is two laser Doppler power spectra obtained by examining humanskin using the apparatus of the present invention;

FIG. 7 shows a typical pulse type flux measurement from a fingertipusing the apparatus of the present invention;

FIG. 8 shows an example of blood perfusion image and video image usingthe 2-D apparatus of the present invention;

FIG. 9 shows a video image and 256×64 perfusion image of a dorsalsurface of a hand using the 1-D apparatus of the present invention;

FIG. 10 is an outline schematic drawing of a non-scanning apparatus fora multi-wavelength imager in which visible and near infra-red scatteringlight is separated by a cold mirror; and

FIG. 11 is an outline schematic drawing of a multi-wavelength imagerwhich can be operated in scanning or non-scanning mode.

FIG. 1 is a perspective view showing schematically an embodiment of thefast perfusion imaging with simultaneous video imaging according to theinvention. A beam of light (A1) from a low noise stable high coherencenear infra-red laser or visible laser, with suitable optics to produce adiverging beam (A2), is reflected from a mirror (A5), which is a hotmirror for infra-red laser radiation or a beam splitter for visiblelaser radiation, and projected onto a section of tissue surface (A4).The hot mirror (A5) has the characteristics of high reflectance forinfra-red and high transmission for visible. The beam splitter (A5) istypically a 70/30 type (70% reflection, 30% transmission). Laser light(A6) reflected and scattered by the section of tissue surface isreflected again by the hot mirror or beam splitter (A5) and is thenimaged onto the photodetector array (A3) via an objective lens (A7) andan optical band pass filter (A8). Meanwhile, visible light reflected bythe large section of tissue surface (A9) is transmitted through the hotmirror or beam splitter (A5) and is imaged on a CCD camera (A10). Bymeans of laser Doppler signal processing (FIG. 4) and video image framegrabbing (FIG. 5), both blood perfusion image and live video image canbe displayed on the monitor simultaneously. Since the photodetectorarray (A3) only images a part (A4) of the tissue surface (A9) beingimaged by the CCD camera, it is very useful to locate the perfusionimage by superimposing the blood perfusion image on the video image(FIG. 8).

FIG. 4 shows:

a CCD camera (1);

a photodetector array (2);

video image and frame grabbing (3);

Doppler signals collection and processing (4); and

Computer for control and display (5).

To scan the beam across the tissue surface the mirror is rotated and itsposition controlled by the means described earlier.

FIG. 3 is a perspective view of an apparatus similar to that illustratedin FIG. 1 except that the apparatus has a means to irradiate the tissuessurface with a laser line beam (C1, C4).

FIG. 3 shows:

a beam of light (C1) from a low noise stable high coherence nearinfra-red or visible laser, with suitable optics to produce a divergingbeam (C2);

a linear photodetector array (C3);

a tissue surface (C4);

a mirror surface or beam splitter (C5);

laser light (C6) reflected and scattered by the section of the tissuesurface;

an objective lens (C7);

an optical band pass filter (C8);

a large section of tissue surface (C9); and

a CCD camera (C10).

An alternative embodiment of the imaging system, intended for non scanmode operation, is illustrated in FIG. 2 where the laser light from theprojector (B1) is shone directly onto the tissue surface (B6) withoutthe aid of a mirror or beam splitter. For near infra-red laser lightirradiation of the tissue surface the scattered light is transmittedthrough a cold mirror (B5) and imaged onto the detector array (B2) by anobjective lens (B3). Additional optical filtering can be provided ifnecessary with a band pass filter positioned in front of the array. Tovideo image the tissue surface at the same time as recording the bloodperfusion image, the surface is illuminated with white light and thescattered light reflected from the cold mirror to the CCD camera (B4).For visible laser light irradiation of the tissue surface the coldmirror is replaced by a beam splitter, typically a 70% transmission 30%reflection type.

All three embodiments of the imager can be operated as multi-wavelengthimagers with the different laser sources selected sequentially. If bothvisible and near infra-red laser sources are used a beam splitter isused to enable perfusion and video images to be recorded at the sametime.

Another alternative embodiment not illustrated of the imaging systemuses a low noise stable high coherence visible laser light in place ofthe near infra-red laser and a front silvered mirror, having highreflectance for the visible laser light, in place of the hot mirror. Thefront silvered mirror is mounted in a mechanism which enables it to bemoved into one of two positions. In one position laser light isreflected to illuminate the tissue site and laser light scatter from thetissue is reflected to the detector optics. In the second mirrorposition a video image of the tissue site can be viewed and recorded.

FIG. 11 is a perspective view showing schematically an embodiment of amulti-wavelength perfusion imager which has means to irradiate thetissue surface simultaneously with laser radiation of two or morewavelengths derived from one laser projector (E1) or the combinedoutputs of two or more lasers, and image sensor and optical filteringmeans (E8) for simultaneously detecting the multi-wavelength scatteredlight and means for generating separate electrical outputs correspondingto each laser wavelength. This has features similar to the systemsillustrated in FIGS. (1) and (3) with the addition of a laser projector(E1) which includes a fibre optic coupler for combining the outputs oftwo or more lasers. A total of four laser outputs can be combined andused relatively easily with this arrangement.

FIG. 11 shows:

a laser projector (E1);

detectors (E2, E5);

optical filters (E3, E6);

lenses (E4, E7);

a detector module (E8);

the tissue surface (E9);

a hot mirror (E10);

projected laser light illumination disc or line shape profile (E11); and

a CCD camera (E12).

The imager illustrated in FIG. 11 can be used in either scan or non scanmode with either a hot mirror, if the laser wavelengths are allinfra-red, or a beam splitter if the laser wavelengths are a combinationof visible and infra-red.

FIG. 10 is a drawing of an alternative embodiment of a multi-wavelengthimager which is intended for operation in non scan mode only. Thecombined visible/near infra-red light scattered from the tissue isfiltered by the cold mirror (D4). The infra-red is transmitted todetector arrays (D3) and the visible light reflected to detector arrays(D2).

FIG. 10 shows:

a laser projector (D1);

detector modules for visible light (sensors, filter and lens) (D2);

detector modules for near infra-red laser light (sensors, filter andlens) (D3);

a cold mirror (D4); and

the tissue surface (D5).

Imaging of a grid of lines printed on card, or a light generated grid,with both the CCD and photodetector array enables a co-ordinate systemcommon to both video and flux images to be established. Standardcomputing techniques can then be used to superimpose the flux image onthe video image. True colour recording of the video image is possible ifthe tissue surface is illuminated with a suitable white light source.

Laser light reflected and scattered from tissue consists of twofractions, one of which is unchanged in frequency and the other of whichhas a Doppler broadened fraction due to interactions with moving bloodcells in the microvasculature of the tissue. The apparatus of thepresent invention is provided with means for processing the electricalsignals, i.e. blood flow related signals, generated by the plurality ofphotodetectors in the array and means for producing an image of theblood perfusion in the tissue being examined from these processedsignals. In conventional analogue processing techniques, thephotocurrent is converted to a voltage, amplified and filtered with anappropriate weighting function (a ω^(½)). A measure of blood flowthrough the tissue can be obtained by squaring and averaging thefiltered signals. An alternative approach is to digitise the signal,calculate the FFT, multiply each spectral component by a weightingfactor (a ω) and sum all components. The basic algorithm that isimplemented to calculate flux values for each channel is:$\text{Flux} = {{\int_{\omega_{1}}^{\omega_{2}}{{{\omega P}(\omega)}\quad {\omega}}} = {\sum\limits_{n_{1}}^{n_{2}}\quad {{nP}(n)}}}$

where ω=2πf and f is a Doppler frequency shift. Typical values of f₁ andf₂ are 20 Hz and 15 KHz espectively.

Each flux measurement will also have dark and shot noise subtracted andwill be normalised to account for different laser powers, surfacereflectivity and tissue optical absorption.

The analogue processing approach has the advantage of simplicity butneeds many more components which makes it very difficult to implementwhen multi-channel blood flow signals generated by the photodetectorarray have to be processed in parallel unless a customized integratedcircuit is used. Therefore, in the present invention we prefer toprocess the signals digitally by using large scale digital signalprocessing (DSP) ICs.

A block diagram (FIG. 5) of the multi-channel Doppler signal collectionand digital processing unit, using by way of example a 16×16photodetector array, illustrates the electronic means of implementingthe algorithm for a plurality of detectors. In order to achieve a highframe rate of perfusion imaging, the whole unit comprises 4 processingmodules, each amplifying and processing 64 channels of Doppler signals.Output signals from 64 to 256 light receiving elements (1) are convertedto voltages and low pass filtered by the I/V and low pass amplifier (2),followed by the voltage amplifier (3) for further increasing themagnitude of the signals. Multi-plexers (4) and a fast A/D converter (5)are employed to convert each channel of analogue signal into digitalprocessing. A DSP (7) is used for data sampling, multi-channel digitalsignal processing and data communication between the unit and the mastersystem which here is a PC.

A fast FFT algorithm using decimation in frequency is utilised in theDSP (7) for flux calculation. For each channel of Doppler signal storedin the memory (6), the time domain digital signals are converted intothe frequency domain by the FFT transformation, and then, as explainedbefore, each power spectral component is multiplied by the correspondingnumber (or frequency) to produce flux output for the correspondingreceiving element. FIG. 6 demonstrates two laser Doppler spectraobtained from two sub-areas of the skin tissue by using the presentapparatus. The curve a) is the result of high blood perfusion and b) isthe power spectrum of low perfusion. FIG. 7 shows a typical pulse typeflux output from one of the receiving elements when directing the laserbeam onto a finger tip.

By using flux outputs from the plurality image sensor, a two-dimensioncolour map of blood perfusion can be formed and displayed on a colourmonitor. FIG. 8 shows a perfusion image (2) and a video image (3)obtained by using the apparatus of the present invention. Differentcolours (1) on the image represent 8 different ranges of flux readings.This is a 16×16 blood perfusion image from knuckles superimposed on acolour video image of a whole hand recorded at the same time.

FIG. 9 shows a perfusion image and a video image obtained by using theembodiment of the apparatus illustrated in FIG. 3 where the lineardetector array has 64 photodetectors. The laser line has been scanned ina step mode across the surface of the volunteer's hand in 256 steps toenable a 256×64 blood perfusion measurement to be made for thecomputation of a 256×64 pixel image.

The perfusion and video images can be displayed separately or with theperfusion image superimposed on the video image. The dc component of thedetected laser light can also be displayed as an image, as can the bloodvolume.

Quantitive information on blood flux, volume and surface reflectivitycan be determined from the pixel values using standard numericalprocessing. Changes due to stimuli can be displayed and measured.

The magnitude of the blood perfusion measurement depends amongst otherfactors on the distance between the imager and tissue surface, so it isnecessary to measure and record this distance. This can be doneautomatically by imaging with the ccd camera or with the laser beam andphotodetector array a simple black and white shape of known dimensions(eg a rectangle) on a card placed on or just in front of the tissuesurface. The pixel size of the rectangle from side to side measures theangle subtended at the imager by this side of the rectangle and hencethe imager/tissue surface distance can be computed.

A look up table of flux/distance variation can then be used to convertthe value of the measured flux to the equivalent value that would havebeen measured at a standard distance.

What is claimed is:
 1. An apparatus for measuring and imaging bloodperfusion in tissue, comprising: a monochromatic laser light sourceproducing a laser light beam; means for shaping the laser light beam;means for directing the laser light beam onto a section of tissuesurface in order to illuminate said surface section; means forcollecting light scattered from the illuminated section; an image sensorcomprising a plurality of photodetectors, each photodetector of thesensor being able to receive collected light from a predetermined subarea of the section of the tissue surface and produce a correspondingelectrical output signal linearly related to the detected instantaneouslaser light intensity; means for processing the electrical outputsignals from the plurality of photodetectors to produce processed outputsignals, said processed output signals comprising: measurements of thepower spectrum of the photocurrents generated in the detection of laserlight scattered from static tissue and Doppler broadened laser lightscattered from moving blood cells; the calculated average Dopplerfrequency shift for each sub area from which scattered light isdetected; the calculated blood volume concentration for each sub areafrom which scattered light is detected; the calculated blood perfusionfor each sub area from which scattered light is detected; andmeasurements of the intensity of the detected scattered light for eachpredetermined sub area; means for producing an image of the bloodperfusion, in the tissue section irradiated, from the processed outputsignals; and an image display means.
 2. An apparatus according to claim1, comprising means for producing images of the blood perfusion in thetissue and means for producing photo images of the tissue surface fromthe spatial variations of the intensity of the detected scattered laserlight.
 3. An apparatus according to claim 1 comprising a means forproducing images of the blood perfusion in the tissue and means forproducing images of the blood volume concentration in the tissue fromthe processed signals.
 4. An apparatus according to claim 1 whichadditionally comprises a means for producing a video image of the tissuesurface which includes the tissue surface irradiated.
 5. An apparatusaccording to claim 4 wherein there is also provided a means forsuperimposing an image of the blood perfusion on the video image on saiddisplay means.
 6. An apparatus according to claim 1 wherein signalfrequency analysis and calculations of blood flow parameters are donewith the aid of large scale digital signal processor integrated circuitsprocessing a large number of channels in parallel.
 7. An apparatusaccording to claim 1 wherein the beam shaping means comprises a lengthof optic fibre which transmits the laser light to a convenient positionwithin the apparatus and emits the light as a diverging beam having abeam front with relatively uniform intensity.
 8. An apparatus accordingto claim 7 wherein the image sensor has its plurality of photodetectorsdistributed and fixed in a regular 2 dimensional array.
 9. An apparatusaccording to claim 1 wherein the beam shaping means comprises an opticalline generator.
 10. An apparatus according to claim 9 wherein the imagesensor has its plurality of photodetectors distributed and fixed in aregular one dimensional linear array.
 11. An apparatus according toclaim 1 with means for optically filtering the collected scattered laserlight prior to detection with a band pass filter.
 12. An apparatusaccording to claim 1 comprising additional monochromatic laser lightsources having different output wavelengths derived from one or morelaser(s) and; means for selecting a laser wavelength for irradiating thetissue surface; means for switching between the laser outputs ofdifferent wavelengths; and means to compare laser Doppler measurementsmade with light of different wavelengths.
 13. An apparatus according toclaim 12 wherein the selecting means and the switching means comprisesan electronically controlled optical fibre switch with a number of inputoptic fibres corresponding to the number of laser light sources used,and one common output optic fibre.
 14. An apparatus according to claim12 comprising means to irradiate the tissue surface simultaneously withone of (a) laser light of two or more wavelengths derived from onelaser, or (b) the combined outputs of two or more lasers from a beamcombining means, and image sensor and optical filtering means, forsimultaneously detecting the multi wavelength scattered laser light andgenerating separate electrical outputs corresponding to each laserwavelength.
 15. An apparatus according to claim 14 wherein the beamcombining means is an optic fibre coupler with the number of inputfibres corresponding to the number of laser sources used, and one commonoutput fibre.
 16. An apparatus according to claim 14 wherein for each ofthe different laser wavelengths there is a corresponding image sensoreach with a suitable band pass optical filter.
 17. An apparatusaccording to claim 16 wherein each sensor means is positioned to one of(a) one side of a beam splitter if a combination of visible and nearinfra-red laser light is used, or (b) one side of a hot mirror if onlynear infra-red laser light is used.
 18. An apparatus according to claim16 wherein the means to optically filter scattered visible laser lightfrom scattered near infra-red laser light is a cold mirror.
 19. Anapparatus according to claim 1 further comprising: means for scanning abeam of the monochromatic laser light over the tissue surface in apredetermined pattern; means for scanning the beam at substantiallyconstant speed across the tissue; means for determining and recording abeam position at any point of the scan; means for halting the beam at apredetermined position or positions during a scan for predeterminedtimes; and means for making laser Doppler measurements at apredetermined position during a scan.
 20. An apparatus according toclaim 1 wherein means are provided to measure imager/tissue surfacedistance, comprising recording a video image of a 2 dimensional shapeplaced at or close to the tissue surface position and measuring thepixel size of one dimension of the recorded image.
 21. An apparatusaccording to claim 1 wherein means are provided to measure theimager/tissue surface distance, comprising recording a photo image of a2 dimensional shape and measuring the pixel size of one dimension of therecorded image.